I do remember I've read somewhere that modern Christians are no longer required to believe in Hell. Like, it's optional. I've now googled but no such info appeared again. Therefore asking knowlegeable people here - which Church document says the Hell is optional if it is really so.

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You're really going to have to be more specific. Many Christian groups have no structure by which anyone could mandate a particular belief such as the existence of Hell. Do you have in mind a particular group?
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jackweinbenderSep 17 '12 at 19:51

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I wonder if you're thinking of "Love Wins" by Rob Bell. Also, just as a side point, the only person qualified to speak for all Christians (i.e. Jesus Christ) has a terribly annoying habit of not always clearly answering the same way to everyone who asks...
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Affable GeekSep 17 '12 at 20:16

Jehovah witnesses don't believe in Hell as a place of torture
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tunmise fashipeSep 17 '12 at 22:24

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Hell is actually a Zoroastrian belief. The Jewish belief was that death is a place of darkness, a pit, etc. A place of no afterlife. Christianity borrowed the idea and changed it slightly. That's why hell as you know it is not in the old testament.
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user1054Sep 20 '12 at 16:09

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If one believes (1) the book of Revelation and (2) what Jesus said about Hell, it's kind of hard to deny the existence of Hell.
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unregistered-matthew7.7Sep 21 '12 at 1:05

As one of the comments mentioned your probably thinking of "Love Wins". In the book, he does not deny the existence of hell as a real place , rather what he is questioning the nature of hell.

So in the same way, to answer your question, I do not believe there is a major branch of Christianity where the existence of Hell is optional, only the nature of what hell is, is being debated (i.e. who goes there, nature of the punishment, does it last forever or just a time, are the damned forgiven at some point or are they completely destroyed, etc).

Any student of the Bible should undertake very seriously to understand the passages in Jas' answer. Do those passages represent the truth? Yes. Is passing a test on those passages a condition of salvation? No.

There's a big difference between what is the necessary condition for salvation, and the body of doctrine a teacher should teach. There are several places in the Bible which offer very simplistic formulas for salvation. For example:

Romans 10:9

if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved

If we take this passage to be literally true, it is hope to the person who just grasped some doctrine, but probably cannot recite and defend doctrine at the level of, say, the Athanasian Creed. This passage also does not say, "Teachers, be satisfied that you're done when these two conditions are met."

As I said above, there is a big difference between teaching something false and not understanding all of doctrine.

1) Biblical evidence:

James 3:1

Not many of you should become teachers, my fellow believers, because you know that we who teach will be judged more strictly.

2) Church history evidence:

Martin Luther's 95 Theses expresses all kinds of anathemas to the hierarchy of the Roman Catholic Church at the time. But very few make any negative reference to the rank-and-file Catholic pew sitters at the time. The worst accusation is that they might trust in the Pope's prescribed works and "lose the fear of God through them."